Chalcogenide glass (pronounced hard ch as in chemistry) is a glass containing one or more heavy chalcogens (sulfur, selenium or tellurium; polonium is also a heavy chalcogen but too radioactive to use). Chalcogenide materials behave rather differently from oxides, in particular their lower band gaps contribute to very dissimilar optical and electrical properties.
The classical chalcogenide glasses (mainly sulfur-based ones such as As-S or Ge-S) are strong glass-formers and possess glasses within large concentration regions. Glass-forming abilities decrease with increasing molar weight of constituent elements; i.e., S > Se > Te.
Chalcogenide compounds such as AgInSbTe and GeSbTe are used in rewritable optical disks and phase-change memory devices. They are fragile glass-formers: by controlling heating and annealing (cooling), they can be switched between an amorphous (glassy) and a crystalline state, thereby changing their optical and electrical properties and allowing the storage of information.